1. Field of the Invention
This invention relates to an optical pick-up device having a diffraction component for recording/reading information on an optical recording medium such as an optical disk, an optical card, or an optical tape.
2. Description of Related Art
In the field of an optical pick-up device for recording/reading information on an optical recording medium such as a Compact Disk (CD), demands for simplification of structure, assembly, or adjustment, and demands for reduction of costs have been increasing in recent years.
FIG. 21 illustrates a background optical pick-up device which employs a holographic diffraction component. The optical pick-up device includes a semiconductor laser light source 511 for emitting laser light having a wavelength of 780 nm, a photodetector unit 531, the holographic diffraction component 553, a mirror 565, a reflective surface 566, a diffraction grating 541 for generating 3-division light beams, and an objective lens 521. The laser light source 511, the photodetector unit 531, and the holographic diffraction component 553 are integrated in a body tube, and are optically adjusted to form a block.
A light beam emitted from the laser light source 511 is transmitted through the diffraction grating 541 for generating 3-division light beams and the holographic diffraction component 553, is reflected by the mirror 565 for bending the optical path, is transmitted by the objective lens 521, converges on a recording pit-surface of an optical disk 101, and is reflected by the recording pit-surface.
The returning light beam thus reflected by the recording pit-surface is transmitted through the objective lens 521, is reflected by the mirror 565, and is incident onto the holographic diffraction component 553, where two kinds of +1st order diffracted returning light are generated by a holographic surface 554 having two different holographic patterns with different pitches. These two kinds of beams are incident on the reflective surface 566 at an angle equal to or more than a critical angle, are thereby reflected by the total internal reflection, are transmitted through the transmitting surface after the total internal reflection, and arrive in the photodetector unit 531. Thereby, information signals, focusing-error signals, and tracking-error signals are detected.
According to the background optical pick-up device as shown in FIG. 21, the laser light source or the photodetector is capable of being substituted to another one having a different specification. Therefore, modification by substitution of the component to another type of high-speed photodetector or by adoption of a new type of semiconductor laser light source can be easily achieved. Further, initial investments are not expensive for manufacturing products according to this embodiment.
Another background optical pick-up device is disclosed in the Japanese Laid-Open Patent Publication No. 3-225636, which employs a birefringent diffraction component having a birefringent crystal for achieving high efficiency of light-utilization. FIG. 22 illustrates the background optical pick-up device which includes a semiconductor laser light source 511 as a light source, a birefringent diffraction grating 541 for separating a light beam 502 emitted from the semiconductor laser light source 511 into three beams, a collimator lens 523 and an objective lens 521, a birefringent holographic diffraction component 555, a quarter-wave plate 625, a 6-division photodetector unit 532 for detecting the diffracted returning light beam out of the optical axis, and a photodetector 533.
The collimator lens 523 and an objective lens 521 are used for an imaging optical system. The light beam from the diffraction grating 541 is collimated into parallel light beam and converges on an optical disk 101 through the imaging optical system. The birefringent holographic diffraction component 555 diffracts and separates the returning light reflected by the optical disk 101 out of the optical axis of the imaging system. The quarter-wave plate 625 is disposed between the optical disk 101 and the diffraction grating 541 or between the optical disk 101 and the birefringent holographic diffraction component 555.
In this optical pick-up device, the birefringent holographic diffraction component is used for simplifying the structure. Further, a uniaxial structure of this optical pick-up device achieves miniaturization thereof and reduction in weight. In addition, reproduction of signals of the optical disk at high efficiency is achieved, when the birefringent holographic diffraction component and the birefringent diffraction grating are employed.
As described above, by combining the semiconductor laser light source, the photodetector, and the holographic diffraction component, an optical pick-up device-is provided having features accompanied by a miniaturized structure, a reduced weight, and a simplified method for adjustment. However, in the background optical pick-up device for recording/reading, there still remain problems as follows.
(1) High efficiency in utilizing light is desired for recording information on an optical recording medium. In an optical pick-up device for reading information on an optical recording medium such as a Compact Disc (CD), which is normally used for reproduction only, there are few problems regarding efficiency in utilizing light. In this case, the focal length of the collimator lens may be long.
In contrast, in the optical pick-up device for recording a re-writable or write-once optical recording medium such as a Compact Disc ReWritable (CD-RW), a Compact Disc Recordable (CD-R), a Digital Video Disc Recordable (DVD-R), or a Digital Video Disc ReWritable (DVD-RW), a collimator lens having a large numerical aperture and a short focal length is frequently used for collecting light from the semiconductor laser light source with little loss, in order to secure a high power of light on the surface of the optical disk.
However, when such a collimator lens having a short focal length is used, a space for disposing a mirror for reflecting the diffracted returning light, etc., becomes narrow. Therefore, the mirror is required to be miniaturized. In this case, mass-productivity is deteriorated due to difficulties in assembling such a miniaturized component accurately.
(2) A large separation angle is required. As described above, in achieving an optical pick-up device capable of recording, the collimator lens having a large numerical aperture and a short focal length, for example, a numerical aperture of 0.3 and a focal length of 10 mm, is frequently used. In this case, for securing a package interval between a laser diode package and a photodetector package within such a short distance, a large diffraction angle of the diffraction grating should preferably be employed, from this point of view.
For achieving the large diffraction angle of the diffraction grating, a diffraction grating having a short pitch should be employed. In this case, however, due to restrictions in fabricating a grating having such a short pitch, it is difficult to form an ideal grating structure having the short pitch.
As a result, a separation property for the polarized light or diffraction efficiency is generally deteriorated, and a SIN ratio of signals is reduced. With this context, there have been limitations in employing a diffraction grating having a reduced pitch, or large diffraction angle, in the background optical pick-up device.
(3) Divergent transmitted light is imposed on aberration due to anisotropy of substrate crystal. When the birefringent crystal substrate such as a thin lithium niobate substrate is disposed in a divergent optical path, because the refractive index depends on a propagation direction of the light, the transmitted light is imposed on aberration.
Therefore, when the birefringent crystal is disposed between the laser diode light source and the collimator lens, it is preferred that the aberration should be suppressed by disposing an additional optical member for suppressing the aberration. However, in this case, the manufacturing cost increases. Further, mass-productivity is deteriorated, because the thin crystal substrate is not easily processed.
FIG. 23A illustrates yet another background optical pick-up device for recording/reading information on a recording surface of an optical recording medium. The background optical pick-up device is explained with reference to FIGS. 23A-23C.
In FIG. 23A, a divergent light beam emitted from a light-emitting portion 513 of the semiconductor laser light source 511 as a light source is transmitted through the diffraction grating 551, and is incident onto the collimator lens 523 which collimates the light beam into a parallel light beam. Subsequently, the light beam is reflected by an upward-reflection mirror 563, is transmitted by a quarter-wave plate 625, begins to converge when it is transmitted through an objective lens 521, and converges as a light spot on a recording surface 103 of an optical recording medium 101 such as a Compact Disc, etc.
The returning light beam, which is reflected by the recording surface 103, is transmitted by the objective lens 521 and the quarter-wave plate 625, is reflected by the upward-reflection mirror 563, begins to converges when it is transmitted through the collimator lens 523, and is transmitted by the diffraction grating 551.
The diffraction grating 551 is a birefringent holographic diffraction component for diffracting the returning light beam, which has a power of diffraction dependent on the polarization of the light. A plane of polarization of the returning light beam after transmitted by the quarter-wave plate 625 twice, or forth and back, is rotated by 90 degrees from the initial state as emitted from the light source. The birefringent holographic diffraction grating 551 is constructed so as not to diffract the light beam from the light source but to diffract the returning light beam. By this diffraction, the returning light beam is separated from the optical path between the light source and the diffraction grating 551. Then, the diffracted returning light beam is reflected by a mirror 565, to be incident on the photodetector unit 531.
The photodetector unit 531 generates focusing-error signals and tracking-error signals on the bases of the detection of the returning light beam, and also generates reproduction signals for reproducing information. Further, by controlling an actuator (not shown) of a servo-system on the basis of the thus generated focusing-error signals and the tracking-error signals, focusing/tracking operation is performed.
As described above, the optical pick-up device which records or reproduces information of an optical recording medium requires a large power of light beam when recording information. Therefore, the efficiency of light utilization from the light source is focused on for an optical pick-up device having such a structure as shown in FIG. 23A.
FIG. 23B illustrates a collimator lens 523 having a long focal length. When the focal length of the collimator lens 523 is long, even if the separation angle t for separating the returning light beam is relatively small, there are few problems in the layout of the mirror 565 or the photodetector unit 531. However, because the emitted light beam from the semiconductor laser light source 511 is a divergent light beam, not a little portion of the emitted light beam is not collected by the collimator lens 523, and the efficiency of light utilization of the pick-up device generally remains in a low level. Therefore, it becomes difficult to perform operation for writing information at a high rate.
When numerical aperture of the collimator lens 523 is increased for utilizing light efficiently, in the optical pick-up device equipped with a collimator lens 523 having a long focal length, the diameter of the collimator lens 523 is also increased, the dimension of the optical pick-up device itself is therefore undesirably enlarged.
FIG. 23C illustrates a collimator lens 523 having a short focal length and a large numerical aperture. In this case, an amount of the light beam collected by the collimator lens 523 is increased, in principle. However, a mirror 565 which reflects the returning light beam toward the photodetector unit 531 is required to be disposed in a position so as not to shield the divergent light beam emitted from a light-emitting portion 513. Therefore, a separation angle ζ should be set considerably larger than the separation angle ξ of FIG. 23B.
In order to increase the separation angle of the birefringent holographic diffraction component as a diffraction grating, a pitch of the grating has to be reduced. This requires, however, adoption of a high-level micro fabrication process, which in turn increases production costs, and by which mass-productivity is deteriorated.
If a diffraction grating having a small pitch, which is produced by a fabrication method without sufficient fabrication accuracy, is employed, then poor quality in transparency or diffraction efficiency may reduce power of the light projected on the optical recording medium or the returning light beam. In this case, a problem may arise, for example, a S/N ratio of the signals generated by the photodetector may be reduced.
Further, in an optical pick-up device which employs a birefringent crystal such as a lithium niobate crystal, a transmitted light beam, as far as it is divergent, is imposed on aberration, because refractive index of the birefringent crystal is dependent on propagation directions of the light. The aberration may be compensated using a compensation optical component, but this further increases costs. In addition, the scale of the optical pick-up device becomes large.
FIG. 24 illustrates still another background optical pick-up device, in which a holographic diffraction component is employed. A laser light beam, which is emitted from a semiconductor laser light source 511, converges on an optical information recording medium such as an optical disk 101, through a holographic diffraction component 553 and an objective lens 523. Then, the returning light beam through the objective lens 523 is diffracted by the holographic diffraction component 553; thereby the returning light beam reflected by the optical disk 101 is separated from the light emitted from the semiconductor laser 511.
A photodetector 531 detects the returning light beam which is diffracted by the holographic diffraction component 553. Information recorded in the optical disk 101 is reproduced on the basis of signals which are obtained through detection of the returning light beam by the photodetector 531.
Due to restrictions in manufacturing a holographic diffraction component having a short pitch of grating, the background optical pick-up devices frequently employ a holographic diffraction component having a small angle of diffraction. As a result, the semiconductor laser 511 and the photodetector 531 are arranged with a very close distance, for example, in a range of 1-2 mm.
In this case, the following shortcomings may arise. First, noise may be superimposed on signals of the photodetector 531, when the semiconductor laser 511 is driven with a high-frequency modulation. This is typical in a photodetectors having a detection circuit therein, and may deteriorate marginal detection of signals. Second, an optical pick-up device is normally equipped with an optical unit which is packed with semiconductor laser 511, holographic diffraction component 553, and a photodetector unit 531. In this case, modification of one unit by substitution of the unit, especially semiconductor laser 511, may not be easy. Therefore, degree of freedom in designing is relatively low in such an optical pick-up device.